Preparation and Analysis of Mesoproterozoic Acritarch Ultrastructure: Novel Techniques In Focused Ion Beam Electron Microscopy

Focused ion beam electron microscopy (FIB or FIB-EM) has been gaining popularity across multiple disciplines over the past decade. Widely utilized as a stand-alone instrument for micromachining and metal/insulator deposition, the high-precision, submicron-scale ion milling/sectioning capabilities of FIB systems have been well documented in the materials science literature. These capacities, along with integrated micromanipulator tools, have promoted the FIB's reputation as a powerful, in-situ, site-specific transmission electron microscopy (TEM) ultrathin foil preparation tool. In addition, recent advancements in the field-emission guns (FEG) of FIB systems have provided spatial resolution comparable to that of many high-end scanning electron microscopes, thus offering potential as an imaging tool in addition to material deposition and removal capabilities. More recently, FIB preparation techniques have been applied to geological samples to explore mineral inclusions, grain boundaries, and microfossils, to name a few. This technology offers considerable benefits over commonly used TEM preparation techniques, including high-precision site-selectivity and minimal time-consuming sample preparation requirements. However, its operational capabilities as a stand-alone analytical instrument remain relatively underutilized outside of materials sciences and semiconductor manufacturing industries. Here, we illustrate a novel method for analyzing organic-walled microfossil ultrastructure using focused ion beam electron microscopy. Our method entails sequential ion-beam-milled cross-sections imaged via integrated high-resolution field-emission scanning electron microscopy, generating a form of in-situ, site-selective, three-dimensional microscopy, or FIB tomography. This technique will provide unprecedented ability to characterize microfossil ultrastructure with important implications for problematic microfossil systematics. The focus of this presentation will be on the elucidation of ultrastructural characteristics and nano-scale details of sphaeromorphic and acanthomorphic acritarchs from the ³1000 Ma Mesoproterozoic Ruyang Group of North China, including Dictyosphaera delicata and Shuiyousphaeridium macroreticulatum, using FIB tomography and three dimensional modeling. Included here will be discussion of three-dimensional characteristics of acanthomorphic processes and vesicularly-enclosed central bodies.